There are the following conventional temperature compensation methods for the secondary resistance of an induction motor: (1) a method in which the variation in the secondary resistance is derived from the variation of the actual output voltage from the target voltage, and this variation is input into the slip angular frequency calculator for compensation (such as Japanese Patent Application No. Hei 1[1989]-267706); (2) a method in which the variation in the secondary resistance is derived from the secondary magnetic flux and the primary current, and this variation is an input of the slip angular frequency calculator for compensation (such as Japanese Patent Application No. Hei 1[1989]-82787); and (3) a method in which the secondary time constant of the motor is calculated from the transient characteristics of the output current or voltage as a DC voltage or current is applied, and the result is input to the slip angular frequency operator for compensation.
However, for said method (1) which detects the output voltage to make the temperature compensation, since compensation is performed when there is a certain change in the output voltage, the compensation has to be performed when the output voltage is high enough (that is, the output frequency is high enough), and so the compensation cannot be performed in the starting state.
For said method (2) which performs temperature compensation by the secondary magnetic flux and the primary current, since detection of the secondary magnetic flux is difficult, and the secondary magnetic flux is calculated; in this case, as the secondary magnetic flux is calculated from the output voltage, the compensation is performed by a certain variation in the output voltage, so the compensation cannot be performed in the start state.
For an elevator motor, however, the control should be performed over the entire region from start to stop, and the compensation has to be performed in the start stage; hence, said methods (1), (2) cannot be used for the temperature compensation of elevators.
On the other hand, for said method (3) which by measuring an output voltage, or the transition characteristics of the output voltage, by applying a direct current, the measurement time is long for a motor with a long time constant. For an elevator, the time from the stopping of the doors to the start of motion of the cage becomes longer, and the passengers feel uncomfortable.
The purpose of this invention is to solve the problems of the conventional methods by temperature compensation of the secondary resistance with a value near the actual secondary time constant, even in the start state, without calculating the secondary time constant.